added ruby 2.0.0 to available rubies
compare ruby versions

to allow for up- and downgrades of ruby this needs to be done in addition to just checking the availability of any ruby
more intuitive alternative priorities

this way the latest ruby version is preferred, using the version number as priority
checkinstall needs to force-overwrite for ruby

otherwise it aborts when finding another version that has been installed previously
Added --upgrade argument

This need to be used to perform potentially destructive tasks - right now we use it for upgrading the current Ruby to a newer release
Merge pull request #21 from gurgeous/ruby-upgrade

Ruby upgrade